High-throughput patterning of photonic structures withtunable periodicityThomas J. Kempaa,1,2, D. Kwabena Bediakoa,1, Sun-Kyung Kimb,1, Hong-Gyu Parkc,2, and Daniel G. Noceraa,2

aDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138; bDepartment of Applied Physics, Kyung Hee University,Gyeonggi-do 446-701, Republic of Korea; and cDepartment of Physics, Korea University, Seoul 136-701, Republic of Korea

Contributed by Daniel G. Nocera, March 12, 2015 (sent for review December 2, 2014)

A patterning method termed “RIPPLE” (reactive interface pattern-ing promoted by lithographic electrochemistry) is applied to thefabrication of arrays of dielectric and metallic optical elements.This method uses cyclic voltammetry to impart patterns onto theworking electrode of a standard three-electrode electrochemicalsetup. Using this technique and a template stripping process, pe-riodic arrays of Ag circular Bragg gratings are patterned in a high-throughput fashion over large substrate areas. By varying the scanrate of the cyclically applied voltage ramps, the periodicity of thegratings can be tuned in situ over micrometer and submicrometerlength scales. Characterization of the periodic arrays of periodicgratings identified point-like and annular scattering modes at dif-ferent planes above the structured surface. Facile, reliable, andrapid patterning techniques like RIPPLE may enable the high-throughput and low-cost fabrication of photonic elements andmetasurfaces for energy conversion and sensing applications.

nanopattern | electrochemistry | photonics | silicon | nanofabrication

Developments in photonics and plasmonics have provided arich array of approaches for coupling and guiding light (1–5)

in optoelectronic and energy applications. The structured sur-faces required for photon management would ideally feature:(i) submicrometer to nanoscale feature size, (ii) precise control offeature size and periodicity over a broad range of length scales,(iii) structures that can be formed over reasonably large substrateareas, and (iv) the capacity to integrate metal and dielectric struc-tures with various substrate geometries. To date, the fabrication ofnanoscale and submicrometer structures has relied on a rich rep-ertoire of patterning and assembly techniques (6–15). Aside fromwell-established techniques such as photolithography, elec-tron-beam lithography, and focused ion beam (6), more recentpatterning and assembly methods include directed and self-assembly (7–9), superlattice nanowire pattern transfer (10),dip-pen lithography (11–13), soft-lithography (14), and elec-trochemical lithography (15). Among these strategies thereare the expected trade-offs between fidelity and resolution. Inaddition, patterning speed, scalability, and the cost/ease ofimplementation are important factors affecting the feasibilityof a given method toward preparing structured surfaces foroptical management.Reactive interface patterning promoted by lithographic elec-

trochemistry (RIPPLE) is a method to form and propagate pe-riodically spaced submicrometer structures over large areas. Wedemonstrate the ability of the method to pattern optical ele-ments with facility while maintaining control over fidelity andresolution to allow for the fabrication of arrays of dielectric andmetallic optical elements. Photonic elements consisting of Agcircular Bragg gratings and large-area periodic arrays of theseperiodically structured elements were prepared and character-ized. The ability to pattern complex submicrometer structuresover large areas in a facile, reliable, and timely manner hassignificant implications for fabrication of photonic elements andmetasurfaces (16, 17) for energy and sensing applications.

Results and DiscussionThe RIPPLE method can rapidly pattern large areas of a sub-strate with periodic nanoscale features composed of oxidic ormetallic species. This method uses cyclic voltammetry to impartpatterns onto the working electrode of a standard three-elec-trode electrochemical setup (Fig. 1A). A parent material, whichcan be a semiconductor (e.g., Ge) or metal (e.g., Cu), is de-posited by chemical or physical vapor deposition over theworking electrode. Subsequently, a thin layer of polymer resist isdispersed over this parent layer and defined with lines or dots,which give the electrolyte access to the underlying parent film.The substrate thus prepared is the working electrode in a stan-dard two-compartment electrochemical cell filled with 0.1 Msulfuric acid as the electrolyte. Details of the substrate prepa-ration and cyclic voltammetry are provided in Materials andMethods. By applying a linearly ramped potential sweep (0.1–1.2 V;all potentials are referenced to the Ag/AgCl electrode) betweenthe working electrode and a Pt mesh counterelectrode, we pro-mote etching of the parent material, which originates at the siteof the lines/dots exposed to solution and then propagates un-derneath the resist.To demonstrate the capabilities of the RIPPLE technique, a

Ge thin film was deposited over a Si substrate and the overlaidpolymer photolithographically defined with an array of 2-μm-wide lines (Materials and Methods). After applying the cyclicvoltammetric potential sweeps to this working electrode andsubsequent stripping of the resist, an atomic force microscopy(AFM) map of a region of one pattern reveals well-defined

Significance

Patterning large substrate areas with arrays of submicrometerstructures in a facile, reliable, and timely manner is importantfor fabrication of optical elements that capture, guide, andconvert light. RIPPLE (reactive interface patterning promotedby lithographic electrochemistry) is an electrochemical pat-terning method that is demonstrated for the rapid fabricationof periodic arrays of metallic circular Bragg gratings over largesubstrate areas. The grating period can be tuned in situ overmicrometer and submicrometer length scales in a high-throughput fashion. We have identified point-like and annularscattering modes at different planes above the structuredsurface, suggesting the potential to use such structures tocontrol the propagation of light. The described methods maybe useful for high-throughput fabrication of sensors and light-management elements for energy conversion applications.

Author contributions: T.J.K., D.K.B., and D.G.N. designed research; T.J.K., D.K.B., and S.-K.K.performed research; T.J.K. contributed new reagents/analytic tools; T.J.K., D.K.B., H.-G.P., andD.G.N. analyzed data; and T.J.K. and D.G.N. wrote the paper.

The authors declare no conflict of interest.1T.J.K., D.K.B., and S.-K.K. contributed equally to this work.2To whom correspondence may be addressed. Email: tkempa@fas.harvard.edu, hgpark@korea.ac.kr, or dnocera@fas.harvard.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1504280112/-/DCSupplemental.

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parallel lines of submicrometer width and height (Fig. 1B, Top).Over the range of cyclic voltammogram (CV) scan rates used inpatterning (50 to 200 mV/s), the features are 20(±1) nm high and360(±15) nm wide. Transmission electron microscopy (TEM)cross-sections reveal the features to have an amorphous mor-phology whereas energy dispersive X-ray spectroscopy (EDS)maps show them to be composed of Ge and O (Fig. 1B, Bot-tom). A feature composed of an oxide of germanium isconsistent with the thermodynamically favored products ofoxidation of Ge at pH 1 and applied potentials used for pat-terning. Aside from linear features, concentric rings can bepatterned easily from dots defined through the resist layer to theunderlying parent material. Cyclic application of voltage rampsleads to lateral etching of the parent material as well as site-specific localization of nanoscale periodic features, the numberof which is defined by NCV – 1, where NCV is the number of CVscans (Fig. 1C). In particular, we found that the period sepa-rating features shows a power-law dependence on CV scan rate

and are able to finely control this spacing from 500 nm to 10 μm(SI Appendix, Fig. S1). We note from an examination of CVsweeps and SEM and TEM images (SI Appendix, Fig. S1) thatpatterning occurs by complete etching of the Ge film to exposethe underlying Si substrate and localization at the interface be-tween the periphery of the unetched Ge film and exposed Sisubstrate. The lateral extent of etching is controlled by CV andelectrolyte conditions, which provide the driving force and pHrequired for formation of the aqueous species of the parentmaterial as predicted by its Pourbaix phase diagram. The local-ization of oxidic Ge ridge pattern (rings and lines) forms at thepoint where the CV scan direction changes from anodic tocathodic sweep. The patterns appear similar to those gener-ated from moving boundary simulations of etching processes(18). Though pattern formation appears electrochemicallymediated, capillary flows leading to contact line pinning anddepinning (19) at the periphery of the unetched parent ma-terial may intervene in the generation of periodic features.

Fig. 1. (A) Schematic of the RIPPLE process. (Left) A standard three-electrode electrochemical setup is used. A working electrode is covered with the parentmaterial (dark blue) to be patterned. Lastly, a top layer of resist (pink) is deposited and prepared with micrometer-scale dots or lines, which give the elec-trolyte access to the parent material. A series of cyclic voltammetric sweeps (NCVs) are performed in acidic electrolyte to periodically pattern the workingelectrode. (Right) Periodic arrays of periodic circular structures are formed rapidly as shown in the optical image. The area beyond the circles is theunpatterned parent film covered by resist. (B, Top) AFM topographical map of two linear features patterned using the RIPPLE process. (Bottom) Bright-fieldTEM image of the axial cross-section of a representative feature as noted with the red box. Dashed line denotes the interface between underlying Si substrateand the deposited feature. EDS maps of Ge and O sampled from within the feature region (light contrast) in the TEM image at left. (C) During cyclic ap-plication of voltage ramps, lateral etching of the parent material occurs in concert with site-specific localization of nanoscale periodic features (light blue).The number of features is controlled by the number of CVs whereas their period is controlled by voltage scan rate. (D) CV data for a patterning experimentlasting eight cycles (for clarity, only cycles 1, 3, 4, 5, 7, and 8 are shown). Solid line, anodic sweep; dashed line, cathodic sweep. Red line is intended as a guide for theeye showing peak evolution from cycle 1–8. (Inset) SEM image of a portion of the concentric rings patterned from a single dot at a scan rate of 100 mV/s.

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The versatility and structural control afforded by the RIPPLEmethod was exploited to fabricate large-area arrays of photonicelements. A resist layer coating a 250-nm film of Ge (parentmaterial) was photolithographically patterned with a 0.25-mm2

square array of 2 × 2-μm dots separated by 60 μm. A typicalpatterning experiment lasting eight CV cycles performed be-tween –0.2 and 1.2 V (vs. Ag/AgCl) at 100 mV/s on the afore-mentioned substrate (working electrode) is shown in Fig. 1D(Materials and Methods). The anodic sweep on the first CV cycleis characterized by a large peak at 0.8 V and subsequent CVcycles show a well-conserved broad peak near this potential. Theamplitude of this broad feature is initially smaller than that ofthe dominant peak of the first CV sweep, but monotonicallyincreases for subsequent cycles. Cathodic sweeps are consistentlyfeatureless compared with anodic sweeps. Emanating from thesite of each predefined dot in the array is a structure consisting ofseven concentric periodically spaced rings. AFM analysis of aportion of one structure reveals a feature height, width, andperiod of 20 nm, 360 nm, and 2.7 μm, respectively (Fig. 1D,Inset). The large peak of the first anodic sweep arises from thelarge mass of Ge that undergoes oxidation to form the etchedregion at the center of each concentric ring structure. The pre-defined dot size defines the diameter of this initial etched regionbefore localization of the first ring. Thereafter, the spacing be-tween rings is dictated by the scan rate and the voltage rangeswept out during CV cycles. The monotonic increase of thebroad peak occurs in concert with expansion of the etched regionbetween periodically deposited rings. The dispersion in periodand feature size is <3% and <5%, respectively. We have pat-terned features with diameters as large as 80 μm. We surmisethat the lateral etching limit is defined by the resist’s elastic

modulus, which, if exceeded, would cause the resist to collapse,thereby restricting the electrolyte filled space.Periodic circular or linear patterns of nonmetallic and metallic

submicrometer structures are of interest as waveguides andplasmonic cavities for sensing and integrated optical applications(20, 21). The features patterned by RIPPLE are characterizedby low surface roughness, high patterning selectivity, and smalldispersion for periods <1 μm, characteristics that are relevant tothe design of high-quality optical gratings. We fabricated a Agcircular Bragg grating (CBG) array by template stripping (22)from a periodic array of Ge concentric ring structures (Fig. 2).The Ge concentric ring master pattern (Fig. 2A) was preparedusing RIPPLE performed over six CV cycles at a voltage scanrate of 290 mV/s to yield structures containing five concentric Gerings separated by a period of 1 μm. A silver film deposited overthe Ge master pattern was subsequently peeled off to reveal anarray of CBGs recessed in the film (Fig. 2 A and B). SEM imagesconfirm that large periodic arrays of intact CBGs, each bearingfive grooves separated by a period of 1 μm, can be templatedfrom the master Ge substrate (Fig. 2C). An AFM profile com-posed of the average of 14 linescans acquired across an ∼10 μm2

area of a CBG shows an average groove depth of 15 nm, width of230 nm, and period of 1 μm (Fig. 2D). These dimensions verifythat the metallic CBG pattern is a reasonable replica of the Gemaster pattern. Notably, the master can be reused multiple timesto template other CBG arrays composed of different metals.To demonstrate the quality of the CBGs, we acquired their

scattering profiles. A sample area that consists of nine CBGsin a square array (Fig. 3 A, 1) was irradiated with the 500-nmmonochromated output of a supercontinuum laser. Images ofscattering profiles (Fig. 3 A, 2 and 3) were acquired at two

Fig. 2. (A, Left) Schematic of a master pattern consisting of a periodic array of periodic concentric rings composed of oxidic Ge. (A, Right) Schematic of a AgCBG array templated from the master pattern. (B) Schematic of the Ag CBG array fabrication by template stripping. (C, Left) SEM image of a portion of the AgCBG array containing 63 intact CBGs. (Right, Top) SEM image of a single CBG. (Right, Bottom) SEM image of the periphery of a CBG showing grooves. (D) AFMline-scan across a CBG verifying that the depth, width, and period of the grooves are similar to the dimensions of the Ge rings in the master pattern.

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slightly different imaging planes within ∼1 μm of the focal plane.These images show that point (Fig. 3 A, 2) and annular(Fig. 3 A, 3) scattering arises from each CBG. In particular,complex “checkerboard” interference patterns are clearly dis-cernible at the interstices between CBGs (Fig. 3 A, 3). Suchstructured scattering is unique to a periodic array of periodicgrating structures, where such surfaces may be interesting can-didates for metasurface applications (16, 17) especially given the ease,scale, and configurability with which RIPPLE can pattern them.To define the origin of the measured scattering modes, we

performed finite-difference time-domain (FDTD) simulations ofAg CBGs. A simulated reflectance spectrum for a single CBGhaving the same dimensions as the patterned structures (Fig. 3B)reveals wavelength-dependent variations in reflected amplitude.Of particular interest is a distinct dip in reflectance observed at505 nm. A simulated near-field x–y plane scattering profile at thiswavelength acquired at a height of 0.5 μm from the top surface ofthe CBG (Fig. 3B, Inset) reveals point scatter from the center ofthe CBG as well as more complex annular scatter from its pe-riphery. A simulation (axial view) of the electric fields normal tothe Ag–air interface of the CBG at 505 nm shows that thesefields propagate with alternating phase along this interface (Fig.3C). This result is diagnostic of the presence of surface plasmonpolaritons, which are excited by interaction of the incident planewave with the periodic grooves of the CBG (23, 24). Together,simulation and theory not only reconstitute the essential featuresof the scatter profiles measured in Fig. 3A, but also suggest a role

for surface plasmon polaritons in the spatial control of scatteringfrom these Ag CBGs.

ConclusionWe have shown that through a combination of the RIPPLE tech-nique and template stripping, periodic arrays of periodic opticalelements can be easily and rapidly patterned over large areas of asubstrate. Detection of point-like and annular scattering modesattests to the fidelity of the patterns and their potential to controlthe spatial profile of light emanating from them. A number ofpromising studies and technologies related to photonics (25, 26),plasmonics (1–5), and metasurfaces (16, 17) are contingent on theability to pattern nanoscale to submicrometer features that areperiodic over large areas, with large-area patterning being the keychallenge. To this end, we view RIPPLE as providing some com-pelling attributes, including the ability (i) to control pattern peri-odicity in situ, with CV scan rate, from <1 μm to several tens of μm,(ii) to pattern rapidly over large substrate areas, and (iii) to patterneasily using low-cost and generic chemicals and equipment.We also note that direct patterning of metallic and dielectric

gratings is possible, thus obviating the need for a templatestripping process. Furthermore, it may be possible to operateRIPPLE in a purely additive mode wherein periodic depositiontakes place from solution-phase precursors delivered via micro-fluidic arrays. High-quality optical elements characterized by lowsurface roughness, sharp pattern profiles, and a small dispersionin submicrometer periods are required to fully capitalize onthe promise of RIPPLE as a fabrication technique for opticalapplications. With a detailed understanding of the patterningmechanism and materials limits, further insights into key pa-rameters dictating resolution and quality will emerge. Forexample, by tailoring the surface tension and viscosity of theelectrolyte solution, we anticipate being able to reduce the size ofpatterned features, to increase the steepness of feature sidewalls,and to reduce surface roughness. In general, our findings es-tablish the viability of implementing RIPPLE-based structures inphotonics and plasmonics, where high-throughput and reliablemethods are needed to pattern dielectric and metallic elementsover large areas.

Materials and MethodsRIPPLE Fabrication. Silicon p-type doped substrates (Prime Grade 3–5 Ω cm,Nova Electronic Materials) of dimension 2 cm2 were cleaned by UV/ozoneashing followed by etching in buffered hydrogen fluoride. A home-builtchemical vapor deposition reactor was used to grow a 250-nm-thick poly-crystalline p-type Ge film at 330 °C at a growth pressure of 28 torr. After Gedeposition, the substrate was coated with a 500-nm-thick layer of S1805photoresist (S1805, MicroChem Corp.) or poly(methyl methacrylate) (PMMA)e-beam resist (PMMA C5, MicroChem Corp.). Using photolithography orelectron beam lithography, 2 μm × 2-μm dots were defined through theaforementioned resists to the underlying Ge film. A portion of the substratewas left free of resist to facilitate electrical contact of this working electrodevia a Cu alligator clip to the potentiostat. The back and sides of the substratewere covered with a lacquer (Microshield, Tolber Chemical) to preventelectrolyte exposure to these regions. A two-compartment electrochemicalcell was filled with 0.1 M sulfuric acid. The substrate (working electrode) wassubmerged into one compartment. iR compensation was performed beforeevery patterning experiment and yielded typical resistance values in the rangeof 500–1,500 Ω. Standard three-electrode cyclic voltammetry experimentswere conducted using a Ag/AgCl reference electrode (BAS Inc.), a Pt meshcounterelectrode, and potentiostat (760D series, CH Instruments Inc.). Fol-lowing cyclic voltammetry, all resists and lacquer were removed by soakingthe substrate in acetone for ∼1 min followed by a 10-s rinse in isopropanol.A gentle stream of N2 enabled drying. For Ge patterning, scans were initi-ated at the open circuit potential, which was 0.1 V.

Preparation of CBG. A 0.25-mm2 square array containing 81 concentric ringstructures was patterned using RIPPLE. Each structure had five Ge rings witha period of 1 μm patterned via six CV cycles at a voltage scan rate of 290 mV/s.A 150-nm Ag film was deposited over this Ge master pattern by electron-beam

Fig. 3. (A, 1) CCD image of nine CBGs arranged into a square array. (A, 2and 3) CCD images of the scattering profiles detected above the CBG arraysurface for 500-nm plane-wave illumination. (B) Simulated reflectancespectrum for a CBG having equivalent dimensions to the fabricated andmeasured one in Fig. 2C. The asterisk denotes the dip in reflectance at 505 nm.(Inset) Simulated near-field scattering profile of a CBG obtained by acquiringthe projection of the Poynting vector on a plane (x–y) 0.5 μm above the CBGsurface. (C, Top) Axial view (x–z plane) of the electric field intensity, plottedas log(E2), normal to the Ag–air interface of the CBG for 505-nm plane-waveillumination. (C, Bottom) Magnified view of the cross-section bounded bythe teal box above. Dashed white line denotes Ag–air interface.

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evaporation (Denton Vacuum LLC) after pumping to a base pressure of 2 × 10−7

torr. Next the Ag film side of the sample was pressed into contact with a cleanquartz slide coated with a thin film of epoxy. The sample was cured overnight.After this, the Ge master pattern was peeled away from the Ag film usingtweezers. The resultant Ag film remained fused to the quartz substrate and borea high yield of recessed concentric ring structures. These recessed structuresconstitute the array of CBGs, which were analyzed in Fig. 3.

Acquisition of Scattering Profiles. To obtain scattering profiles from a CBGprepared by RIPPLE, a broadband (λ = 450–800 nm) supercontinuum laser(EXB-6, NKT Photonics) was used as an illumination source. A spectrometer(SpectraPro 300i, Acton Research) was used to select a narrowband illumi-nation with a central wavelength of λ = 500 nm. The scattering images werecaptured by a Si charge-coupled device whereas the CBG specimen wastranslated through the focal plane of a 10× optical lens.

FDTD Simulations. FDTD simulations were used to calculate the near- and far-field scattering profiles from a CBG. A nonuniform grid method was used toaccurately model light propagation in our experiment. To this end, we used a

spatial resolution of 1 nm at the metal–dielectric interface and 5 nm elsewherein the simulation domain. The dispersive optical constants of silicon and silverwere obtained by fitting their measured refractive index and extinction co-efficient over the wavelength range λ = 400–800 nm. The reflectance spectrumfrom interaction of a normally incident plane wave with the CBG was acquiredfrom 400 to 800 nm in 5-nm increments. The total-field scattered-field methodwas applied to ensure that the CBG experiences an infinitely extended planewave. The near-field profile was calculated by obtaining the normal Poyntingvector at a plane 500 nm away from the top surface of a CBG.

ACKNOWLEDGMENTS. D.G.N. acknowledges support of this work byNational Science Foundation Center for Chemical Innovation Center CHE-1305124. S.-K.K. acknowledges support of this work by Basic ScienceResearch Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, Information/CommunicationTechnology & Future Planning (NRF-2013R1A1A1059423). H.-G.P. acknowl-edges support by an NRF Grant (2009-0081565) funded by the Koreangovernment (Ministry of Science, Information/Communication Technol-ogy, and Future Planning). We thank TomKat Trust for funding of the First100 Watts Project.

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